I melted bitter chocolate (40%) via bain-marie to make chocolate-covered strawberries. As the melted chocolate dried out before I finished dipping all the strawberries, I tried to melt the rest again with the same method but it became sort of muddy and dry. (I didn’t have any problem with milk chocolate.)

What could be done (other than repeating the process with small amounts) to "re-melt" it without losing the bitter taste?

2 Answers
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Chocolate is a sol, consisting of solid particles suspended in cocoa butter. It is something similar to a hard emulsion. And it can separate just the way a liquid emulsion does (think mayonnaise). This happens when you melt the cocoa butter completely, so the solid particles separate from the fat. If it happens to a chocolate bar, your chocolate looks grey. If it happens to a bowl of melted chocolate, the chocolate seizes the way you describe it. This happens with both milk and dark chocolate. If you haven't experienced it with milk chocolate before, you either had luck, or your milk chocolate was of a lesser quality than the dark one and contained non-cocoa fats and/or emulsifiers, which change the behavior of the sol.

The only way to prevent seizing is to work within the correct temperature zone, which is extremely narrow (2-3°C). Even as an experienced confiseur, it is extremely hard to judge it intuitively. If you insist on trying to watch the chocolate and guess when it is OK, you will have inconsistent results, with a seizing once every few tries.

What you need is to get a candy thermometer. Keep it in the chocolate and, whenever the temperature nears the danger zone, put the inner bain marie vessel in a basin of cold water you keep near the stove for this purpose. It will cool rapidly and stay tempered. It isn't a problem if you cool it off so much it hardens again; you can remelt chocolate as often as you want as long as you never exceed the seizing temerature.

And now for the numbers. All kinds of chocolate (milk, white, bitter) harden at 27°C. Between 27°C and 30°C, they are soft, but unworkable, because they are too viscous (hold unpacked chocolate pieces in your hand for a while to see what I mean). The workable zone is 30°C to 32°C for milk (and white) chocolate, and 30°C to 33°C for dark chocolate. Above this, your cocoa fat melts and the chocolate seizes. So, keep an eye at the thermometer, as you see, the zones are narrow.

Edit: I just noticed that you call 40% "bitter". This is a very low cocoa percentage, and I wouldn't let it go up to 33°C. The numbers for "bitter" are probably safe for 70% cocoa and above.

out of curiosity, have you ever tried using an IR thermometer? I have one--but not a candy thermometer--and I wonder if it would do the job sufficiently
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Eric HuDec 15 '11 at 2:53

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@EricHu I haven't tried, but my hunch is that it won't help much. There is a vertical heat gradient in your chocolate, and the top surface will be colder than the chocolate near the bottom. Still, I'll try to remember to check the actual difference next time I melt chocolate.
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rumtscho♦Dec 15 '11 at 12:23

What I do when I want to melt chocolate that I want to stay melted and at the same consistancy is use a double boiler. I let it melt then set the stove to the very lowest so that it will still get consistant low heat to keep it melted while I do my dipping.

What is happening to your chocolate is probably that it is seizing when you melt it the second time. Either that or you are microwaving it and making it lose too much moisture.